High performance CMOS, BiCMOS, bipolar, and complementary BiCMOS process flows often require the dry etching of silicide/polycide films to produce sub-micron low-resistance gate electrodes and local interconnect structures. The etching is required to be highly anisotropic in order to obtain controlled critical dimensions for the devices. For example, the patterning of fine geometry polycide gates for high performance CMOS devices, the formation of narrow local interconnects which are stable at high temperatures, and the formation of fine geometry electrodes for high frequency bipolar devices all require that a "stack" of material layers which include an insulator and a silicide or polycide (silicide plus polysilicon) film be etched. This etching needs to be highly anisotropic to prevent the undercutting of the silicide/polycide film and a resulting loss of critical dimension.
However, most currently available dry etch processes are not sufficiently anisotropic. Lateral etching of the silicide film with respect to the underlying or overlying films occurs, resulting in a loss (reduction) of the critical dimension. In addition, the severe undercut of a silicide film which is covered with an insulator can lead to the formation of non-ideal sidewall spacers which can result in residual metal stringers after a metallization step.
For example, an etch chemistry composed of Cl.sub.2 and CF.sub.4 has been used to etch a stack consisting, from top to bottom, of a resist layer, an insulator, a silicide film (typically tungsten silicide, WSi.sub.x), and polysilicon, all on a silicon substrate. However, this etch chemistry has several disadvantages. It results in excessive undercutting of the silicide layer, which in turn results in non-ideal sidewall spacer formation which causes metal stringers. It also provides poor control of the critical dimension, and in-sufficient lateral diffusion from the silicide layer. This reduces the lateral link between that layer and the remaining portions of the device structure when a polycide stack is used as a contact electrode, as in the case of a double polysilicon self-aligned bipolar device.
For such structures, the Cl.sub.2 and CF.sub.4 based dry etch has been used to etch a resist, insulator, silicide, polysilicon stack which is arranged so that it is partially over a silicon substrate and partially over an oxide layer which overlays the substrate. In this situation, the endpoint of the etch will be based on the polysilicon-oxide interface. This may lead to over-etching of the silicon substrate in those regions in which the stack is arranged over the substrate. This should be minimized to reduce damage to the active device regions formed in the substrate.
U.S. Pat. No. 4,473,436, issued Sep. 25, 1984, naming Beinvogl as inventor, discloses a method for etching a stack which includes metal silicide and polysilicon films arranged over a silicon substrate. The reactive ion etch described in the '436 patent involves the use of a gas mixture which contains fluorine and chlorine as the reactive etch gas. In the preferred embodiment, a gas mixture containing SF.sub.6 and Cl.sub.2 is used to etch a stack which includes tantalum silicide. The SF.sub.6 :Cl.sub.2 ratio and other operational parameters are varied to obtain anisotropic etching of the stack or varying degrees of under-etching or excessive lateral etching of the different layers of material. In the examples shown in the '436 patent, some amount of undercutting of the silicide layer occurs. The '436 patent also fails to disclose an etch process by which a sidewall profile with a desired degree of tapering may be obtained without undercutting of the silicide layer.
U.S. Pat. No. 4,443,930, issued Apr. 24, 1984, naming Hwang et al. as inventors, discloses a method of forming a silicon rich metal silicide layer on a doped polysilicon layer or silicon substrate. A plasma etch process involving CF.sub.4 and O.sub.2 as the etchant gases is described for use in etching a tungsten silicide film (WSi.sub.2). The '930 patent describes the CF.sub.4 :O.sub.2 etch chemistry as being highly anisotropic. However, the inventors of the present invention have found that the CF.sub.4 :O.sub.2 etch chemistry causes lateral undercutting of the silicide layer which results in a loss of critical dimension.
U.S. Pat. No. 5,201,993, issued Apr. 13, 1993, naming Langley as inventor, discloses a method for anisotropically etching an oxide/silicide/polysilicon sandwich structure. The etch process consists of two steps: an oxide etch and a polycide etch. The oxide layer is etched by a plasma formed from a mixture of C.sub.2 F.sub.6, CF.sub.4, and CHF.sub.3, along with He as a carrier gas. The silicide and polysilicon layers are then etched using a plasma formed from Cl.sub.2, again with He as the carrier gas. The '993 patent notes that the production of near vertical etch profiles is assisted by the creation of fluorocarbon polymers which protect the vertical sidewalls from the etching species. The '933 patent notes that the degree of anisotropy of the etch can be varied by altering the ratio of the polymer producing gas (in this case CHF.sub.3) to the fluorine producing gas (in this case CF.sub.4). However, a disadvantage to this etch method is that variation of the degree of anisotropy is achieved at the cost of a change in the etch rate.
U.S. Pat. No. 4,414,057, issued Nov. 8, 1983, naming Bourassa et al. as inventors, discloses a method for anisotropically etching a stack consisting of a lower dielectric layer, an intermediate polysilicon layer, and an upper silicide layer. The silicide layer is etched using C.sub.2 ClF.sub.5 in a reactive ion etch process. The polysilicon layer is then etched using a plasma formed from a mixture of chlorine and fluorine, obtained for example from C.sub.2 F.sub.6 and Cl.sub.2.
Finally, "Fabrication of WSi.sub.x Micron Structures Using RIE in SF.sub.6 --N.sub.2 Mixture", C. Meiqiao et al., Chinese Journal of Semiconductor, discusses the use of SF.sub.6 --N.sub.2 and SF.sub.6 --Ar chemistries for use in the reactive ion etching of WSi.sub.x. However, the article fails to discuss the etching of polycide structures or certain of the etch parameters needed to ensure reproducibility and control of the etch process, such as the etch endpoints. The article also fails to note the significance of using a chemistry which generates a protective polymer in order to obtain vertical etch profiles.
The art thus discloses several etch chemistries which may be used to etch silicide/polycide films. However, none of the suggested etch chemistries appears to satisfy the need for a highly anisotropic etch which does not produce undercutting of the silicide layer, and in fact may be used to increase the critical dimension of the etched layers. In addition, none of the etch chemistries appears to be capable of being controlled so as to produce a sidewall profile having a desired degree of taper. This is beneficial in forming self-aligned contact structures.
What is desired is a method for anisotropically etching a tungsten silicide or tungsten polycide structure which is highly reproducible and results in minimal undercutting of the silicide film. It is also desired to have an etch method which can be varied to produce a desired degree of sidewall taper. These and other advantages of the present invention will be apparent to those skilled in the art upon a reading of the Detailed Description of the Invention together with the drawings.